Many work machines, such as earthworking machines or the like, include fluid actuators, such as hydraulic cylinders, which may be used by the earthworking machines to lift, lower, or otherwise move earthworking equipment. Such fluid actuators may experience many extension-retraction cycles during a work period. For example, a hydraulic cylinder on an earthworking machine may be used to periodically lift and lower a work implement. The work implement may be raised by applying pressurized fluid to the hydraulic cylinder, and the work implement may be lowered under its own weight by releasing the pressure supplied by the fluid. Again, the work implement may be raised by applying pressurized fluid to the cylinder, and again the work implement may be lowered by releasing the fluid from the cylinder. Each time the work implement is raised, potential energy is created within the work implement system, and each time the work implement is lowered by releasing pressure from the cylinder, the potential energy is lost.
In order to reduce energy losses associated with the cyclical lifting and lowering of a work implement, various devices have been proposed to (i) recover and store some of the energy that is released when the work implement is lowered, and (ii) subsequently use the stored energy to raise the work implement during its next lift cycle. For example, in an article entitled “An Energy Recovery System for a Hydraulic Crane,” Xingui Liang and Tapio Virvalo proposed an energy recovery system for reducing energy losses associated with the operation of a crane. Xingui Liang & Tapio Virvalo, An Energy Recovery System for a Hydraulic Crane, Proceedings of the Inst. Mech. Eng'r Part C, J. Mech. Eng'g Science, Vol. 215, no. 6, 737-44 (2001). The proposed Liang system includes a hydraulic lift cylinder connected with the joint of a crane. The lift cylinder is fed by a hydraulic pump, which supplies pressurized fluid to the lift cylinder for lifting the crane. In addition, the proposed system includes two additional assistant cylinders connected with an accumulator. The assistant cylinders share the load of the crane with the lift cylinder. When the boom is lowered, the assistant cylinders charge the accumulator. When the boom is to be raised, the hydraulic pump feeds pressure to the lift cylinder and the accumulator feeds stored pressure back to the assistant cylinders.
Prior systems may suffer from various disadvantages. For example, adding additional separate cylinders to a lift system may increase the cost of the lift system. Moreover, application of additional cylinders to an existing lift system may not be feasible due to space, configuration, or other design constraints. Further, the additional cylinders in prior proposed systems may be constrained to receiving supply pressure from an accumulator and may, therefore, be limited to applying only stored energy to the lift system. Thus, the amount of lift force provided by such additional cylinders may be limited by the pressure storage capacity of an associated accumulator.
The present invention is directed to overcoming one or more disadvantages associated with prior fluid actuating systems.